1. Field of the Invention
The present invention relates to a SAW element, etc. to be used for mobile communication apparatus, etc.
2. Description of the Prior Art
In recent years, higher performance of communication equipment is in progress as a result of evolution and growth of mobile communication technology. These equipment always requires devices such as high-frequency filter and resonator, etc., and these devices are in need of higher performance. Conventionally, SAW elements are in broad use for these devices.
Characteristics of a SAW element are mainly determined by piezoelectric substrate in which surface acoustic waves propagate. What is important as characteristics of a piezoelectric substrate is the electromechanical coupling coefficient and temperature dependency. The electromechanical coupling coefficient is quantity related to pass band width of the filter configured by a SAW element and Q value of the resonance device, and temperature dependency relates to variation quantity of center frequency of the filter for temperature changes, and variation quantity of resonance frequency of the resonance device. In addition, the electromechanical coupling coefficient as well as temperature dependency is intrinsic to material of piezoelectric substrate and the direction of substrate.
The characteristics required for the piezoelectric substrate forming a SAW element are high electromechanical coupling coefficient, for example, so as to secure broad passing band for the high frequency band, and small temperature dependency so as to control frequency variations. However, as for existing piezoelectric substrate, temperature dependency is intensive for those with higher electromechanical coupling coefficient. Therefore, realization of a SAW element with higher electromechanical coupling coefficient and small temperature dependency is regarded as a problem in filter designing.
A PCN system, which is one of mobile communication system, will be described as an example. The PCN system works within a narrow frequency range of 20 MHz for the transmitting frequency band in a high-frequency band and the receiving frequency band. Considering production deviation on elements as well as frequency variation quantity due to temperature changes, in filter designing for a high-frequency band, the frequency range for the transmitting frequency band filter and the receiving frequency band filter will become further narrower.
Accordingly, it will become difficult to secure attenuation in receiving frequency band filter for transmitting band, and attenuation in transmitting frequency band filter for receiving band, or so called interband attenuation. In use of 36.degree. rotated Y cut X propagation lithium tantalate as a piezoelectric substrate, the electromechanical coupling coefficient is 5% and the temperature dependency (temperature coefficient of delay time; TCD) is 35 ppm/.degree. C., and therefore the substantial band width becomes ten and several MHz so that it will not become possible to secure any sufficient interband attenuation. Accordingly, desired is a piezoelectric substrate with around 5% of or more electromechanical coupling coefficient in order to secure a frequency band width, and with TCD of less than 35 ppm/.degree. C. in order to secure interband attenuation.
However, it would not give rise to any problems if there were a piezoelectric substrate with larger electromechanical coupling coefficient, and smaller TCD, but there are not such a substrate among existing substrates. Therefore, a method to reduce TCD in an existing substrate has been proposed.
For example, as shown in IEEE Transactions on Sonics and Ultrasonics, volume SU-31, pp. 51-57 (1984), it is known as a method that a silicon oxide thin film layer with reversed code in that material temperature coefficient is formed on a lithium niobate substrate so as to improve the temperature dependency in propagation characteristics of surface acoustic wave.
However, in the IEEE method, it is necessary to make thickness of the silicon oxide thin film layer extremely thin such as to within a single length of a wave for the wave length of the working SAW at largest in order to attain improved effect on the temperature characteristics. However, it is difficult to make thickness of such thin silicon oxide film layer as well as the film quality thereof homogeneous.
A method to improve temperature characteristics is shown in Japanese Patent Laid-Open No. 6-326553 specification as the one to solve these problems. This method involves a using substrate which has undergone lamination by direct bonding of substrates with respectively different thermal expansion coefficient, wherein, compared with the case with a single substrate, the substantial thermal expansion coefficient of laminated substrates is reduced, and consequently temperature dependency of SAW element is improved.
Conventional SAW element based on the temperature characteristics improvement method shown in Japanese Patent Laid-Open No. 6-326553 will be explained as follows.
FIG. 11 is a sectional view showing a conventional SAW element using a laminated substrate by way of direct bonding. In FIG. 11, a propagation substrate being a piezoelectric substrate is numbered as 31, an auxiliary substrate using low temperature expansion coefficient material is numbered as 32, and comb-shaped electrodes are numbered as 33.
The propagation substrate 31 and the auxiliary substrate 32 are brought into direct bonding. As the propagation substrate 31, lithium tantalate and lithium niobate are used. Normally, thickness of the propagation substrate 31 is not less than five times of the working wave length. Since the thermal expansion coefficients for the propagation substrate 31 and the auxiliary substrate 32 are different, the substantial thermal expansion coefficient of the stuck substrate will be different from the thermal expansion coefficients intrinsic to the respective substrates, and consequently temperature dependency thereof will become different.
However, the conventional SAW element has problems as follows.
A conventional SAW element comprises a propagation substrate and an auxiliary substrate, the both substrates being brought into bonding in the entire surfaces. In this case, it would go well if substrate bonding were complete, but if the substrate bonding is not sufficient, stress to be extended over the surface layer of the propagation substrate will not become homogeneous due to temperature changes. As a result, while the SAW is being transferred on the substrate, the speed of the SAW changes. In addition, in some cases, SAW propagation loss will increase.